Starscream is a hero on the wrong side. Over the centuries he's come to believe both Optimus and Megatron have lost sight of what is best for the Cybertronian race, and simply pursue their own agendas. At some point, he came to the decision that, for the good of the species, a new leader must emerge, and he would be that leader. However, over time, he became as corrupt in his own way as Megatron, although he prefers subtlety and deception to Megatron's brute force. Now, what may once have been noble goals are buried under layers of self-interest, transforming Starscream into that which he supposedly despised.
"Truly the universe favors those who persist."
Starscream, Ghosts of Yesterday
http://tfwiki.net/wiki/Starscream_(Movie)